Circuit boards are in widespread use throughout the electronics field. As is well known, a circuit board comprises a flat board of insular material on at least one side of which is bonded a thin layer of conductor which is typically etched to some desired conductor configuration which interconnects a number of electronic parts which are mounted thereon.
Electrical access to such boards takes a variety of forms. Where only a small number of access points are required such access may be by means of small pins which are mechanically bonded to the board and which are electrically connected to the conductor at the desired point.
Modern circuit boards, however, particularly those utilized in connection with advanced and sophisticated technology, may require electrical access to numerous points on a particular board. For example, digital electronic systems may be constructed of a number of boards on which are mounted a dozen or more integrated circuit chips, each such chip containing an intricate circuit comprised of a very large number of electrical components. A single board may thus contain a vastly complex circuit or number of circuits which may require access to a very large number of points.
One way such multiple electrical access is effected, and at the same time conveniently insertable and removable mechanical support for the circuit board is provided, is by means of circuit board connector strips. These connector strips typically take the form of long, hollow, plastic constructions having a long, slot-like opening for receiving the end of a circuit board. The strips are mechanically connected to a support board, sometimes referred to as the "mother board".
Within the connector construction may be placed a series of contiguous chambers, separated by divider walls. These chambers are adapted to receive resilient conductive contacts such that when the end of a circuit board is inserted into the opening of the connector strip the resilient conductors come into contact with one or both sides of the board.
The circuit board, in turn, has etched thereon at the insertion end a linear series of conductive "islands" which, when the board is so inserted, provide the electrical contact surface for the resilient connectors disposed within the chambers of the connector strip.
As is well-known, the above-described arrangement provides a convenient, low-cost and effective means for connecting circuit boards having large numbers of electrical access points to the electrical system within which it functions. It also provides easy insertion and removal of the board for ease of system construction and of maintenance.
Problems can arise, however, with the use of such an arrangement. For example, highly sophisticated and complex electronic systems may require that several, or many such boards be utilized to make up the entire system. Typically in such cases, the mechanical configuration of this system is designed to utilize space with maximum efficiency. This results in the connector strips being placed in fairly close proximity. In addition, because of the identical appearance of integrated circuit packages, and their typically ordered, linear arrangement on such circuit boards, it becomes possible to mistake one board for another without close and careful inspection. Also, it is possible to simply forget which board goes to which connector.
The consequences, however, of inserting such a board into the wrong connector strip can be disastrous to that board or others. Excessive voltages may be improperly placed on components which, in turn, can cause their destruction. This gives rise to costly troubleshooting and replacement of destroyed parts.
To prevent such occurrences, connector strips are provided with small indexing slots along the edge of the opening of the connector strip. Thin strips of plastic are placed in one or more of these slots to serve as indexing keys. Corresponding grooves are cut into the end of the circuit board to match with the placement of the key or keys. By placing the aligning keys in slots at different locations along the strip, a number of boards/connector strip combinations may be individually keyed to prevent the inadvertent placement of a circuit board in the wrong connector strip.
Certain problems can arise, however, in association with the use of such indexing slots and keys. These problems arise from the very small size of the key. For example, typical dimensions of the cross-section of such a key are of the order of approximately 20 mils by 50 mils. These keys are therefore vulnerable to breaking if sufficient forces are applied to a wrong circuit board being forced into the slot.
In practice, the forces required to insert a circuit board into such a connector strip are considerable due to tolerance variations in the thickness of the board and the width of the connector strip opening, as well as frictional forces with the resilient electrical contacts. In fact, technicians constructing practical systems including such circuit board and connector strip arrangements frequently find it necessary to resort to the use of rubber mallets and the like to get the boards to enter the connector strip opening. In addition, "card extractors" are frequently employed which attach to the physical structure adjacent the end of the connector strips to provide lever action for the insertion and extraction of printed circuit boards. Such extractors are capable of providing insertion and extraction forces of the order of magnitude of 100 pounds. The forces provided by such extractors, and by rubber mallets and the like, are far greater than the minimum necessary to cause the physical destruction of an indexing key such as described above.
As a result, such indexing keys do in fact shatter when an improper circuit board is forced into a connector strip. Not only does this defeat the purpose of the indexing key but the shattered debris of the broken key can lodge in unwanted places and give rise to problems. For example, a piece of broken indexing key might lodge itself between resilient contact and the circuit board thus preventing electrical connection.
The present invention provides an improved circuit board connector strip indexing key which not only fits in the very small indexing slots on circuit board connector strips, but in addition provides sufficient structural strength to avoid shattering and the problems associated therewith.